Turbo machinery such as high performance gas turbine engines have a compressor and turbine which each include one or more annular banks or rows of axially spaced fixed stator vanes which are positioned between rows of rotatable rotor blades. Each rotor blade is formed with a rotor tip, an air foil and dovetail-shaped base or root which mounts within a mating, axial slot formed between adjacent dovetail posts on the web or rim of the rotor disk. The connection between the dovetail root of the rotor blade and the axial slot between adjacent dovetail posts in the rotor disk prevents radial and tangential movement of each rotor blade relative to the rotor disk.
In order to prevent axial movement of the rotor blades, i.e., along the longitudinal axis of the rotor disk and engine, one or more blade retainers are mounted adjacent the axial slots in the rotor disk. These blade retainers must be secured to the rotor disk to maintain the rotor blades in place and yet be easily removable in order to replace the rotor blades.
The most common method of securing blade retainers to the rotor disk is to employ bolts and nuts which are circumferentially spaced about the rotor disk. Bolts provide a strong connection between the blade retainers and rotor disk, but a number of problems are presented. The bolt holes formed in the blade retainer and rotor disk create localized stress areas which reduces the cyclic life of such parts. This is particularly true in view of the high temperatures and high speeds at which the rotor disks and rotor blades are operated within high performance gas turbine engines.
In order to reduce such localized stresses, some prior art designs have incorporated additional material in the areas where the bolt holes are formed on both the blade retainers and the rim of the rotor disk. Although this tends to reduce localized stresses, such addition of material increases the overall weight of not only the blade retainers, but the rotor disk. Moreover, the high strength forgings which are used to fabricate the blade retainers present a difficult machining operation typically requiring the use of electrochemical machining.
The use of bolts to secure axial blade retainers to the rotor disk also presents installation and performance problems. A relatively large number of circumferentially spaced bolts and nuts must be installed to mount the blade retainers in place, and then removed to replace rotor blades. Additionally, the bolts must be carefully torqued in order to avoid overstress at the connection which also increases installation time. Bolt heads and nuts which protrude from the rim of the rotor disk increase the temperature of the surrounding air and create a disturbance of the air flow passing across the disk, i.e., "windage", both of which result in decreased engine performance.
In recognition of the problems identified above, boltless blade retainers have been developed such as disclosed, for example, in U.S. Pat. Nos. 3,768,924 to Corsmeier et al; 4,171,930 to Brisken et al; and, 4,304,523 to Corsmeier et al, all assigned to of the type disclosed in these patents, and others, reduce the problems of localized stress concentration in the rotor disk rim and blade retainers, reduce installation time and difficulty, reduce the weight of the blade retainers and rotor disks and, in some cases, reduce cost. Nevertheless, some problems have yet to be overcome.
For example, in some designs such as shown in U.S. Pat. No. 3,768,924, boltless blade retainers are formed with a plurality of tabs on a radially inner portion thereof which interlock with a plurality of slots machined on the rotor disk. This arrangement provides an effective means to secure the blade retainer to the rotor disk, but the fabrication of the tabs and slots requires a relatively large amount of machining which can increase costs.
Other types of blade retainers such as shown in U.S. Pat. No. 4,171,930 employ clips or shear wires to secure the blade retainer to the rotor disk. While also effective in providing a secure connection between the blade retainer and disk, such clips or wires often protrude into the air flow through the compressor or turbine of a turbo machine. This can create a windage problem and also increase the temperature of such air, thereby reducing engine performance.
It is therefore preferable to remove the structure which secures the blade retainer to the rotor disk from the path of air passing through the compressor or turbine of the turbo machine. As shown, for example, in U.S. Pat. No. 4,304,523, boltless blade retainers have been designed which are held in place by a retaining member carried within a recess or slot formed in the rotor disk. The retaining member is wedged within the recess between the blade retainer and a portion of the rotor disk so that the blade retainer is held in an axially fixed position relative to the rotor disk.
While boltless blade retainers of the type described in U.S. Pat. No. 4,304,523 have the advantage of reducing windage effects, in such designs the retaining member must be moved to a seated, locked position with respect to the blade retainer and rotor disk in order to ensure that the blade retainer is securely locked in place on the rotor disk. No fail-safe structure is provided to ensure that the retaining member has been moved to a locked position, and thus it is possible that the blade retainer could be held in position but not locked in place if the
assembly operation is not performed correctly.